Variable resistance devices find many uses in electrical equipment. One common use of a variable resistance device is as a potentiometer to control volume in, for example, radios or hearing aids.
When used in a hearing aid, a potentiometer must meet a number of requirements. The foremost requirement is size. As hearing aids continue to decrease in size, their components must, necessarily, also decrease in size. This poses a problem for potentiometers which typically rely on electro-mechanical structures to provide variable resistance.
Many of the devices use a center screw around which one or more of the components rotate. As a result, the torque is concentrated and can lead to failure if excess torque is applied to the device.
Another problem faced in conjunction with hearing aids is that repair of the wide number of devices, each using different components, requires that repair centers stock a wide range of different potentiometers. As a result, the cost to repair the hearing aids is increased due to additional overhead associated with those stocks.
Known potentiometers also are typically constructed of a relatively large number of small parts, increasing the difficulty in assembly and, correspondingly, the cost of the devices. In addition, the reliability is impaired because of the likelihood that each of the many parts presents an opportunity for failure in the device.
One example of a known potentiometer is disclosed in U.S. Pat. No. 4,032,880 to De Michele et al. The device comprises a conductive ball trapped between a smooth resistive plate and a grooved conductive plate. The plates are rotated relative to each other to cause the ball to move on the resistive plate, thereby varying the resistance.
This device suffers from the problems discussed above. Namely, it is constructed from a relatively large number of components and uses a center screw connection which concentrates stress. In addition by using a smooth resistive plate, the ball can potentially stick or spin in place on the resistive plate, causing erroneous output.